This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Measurements using near infrared spectroscopy (NIRS) are abundant in muscle tissues, yet very little has been studied in terms of how optical properties changes in normal muscle tissues. We propose to study how various factors influence the optical properties of muscle tissues: (1) How do muscle optical properties vary with age, gender, and muscle fitness level? (2) How much do muscle optical properties vary on different timescales (daily, weekly)? (3) How do changes in muscle optical properties indicate response to exercise training (both short term and long term)?